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Abstract:

Techniques for controlling current flow in semiconductor devices, such as
LEDs are provided. For some embodiments, a current-guiding structure may
be provided including adjacent high and low contact areas. For some
embodiments, a second current path (in addition to a current path between
an n-contact pad and a substrate) may be provided. For some embodiments,
both a current-guiding structure and second current path may be provided.

Claims:

1. A light-emitting diode (LED), comprising: a substrate; an LED stack
for emitting light disposed above the substrate, wherein the LED stack
comprises: a p-type semiconductor layer; and an n-type semiconductor
layer disposed above the p-type semiconductor layer, wherein the LED
stack provides a first current path for the LED; and a second current
path for the LED different from the first current path.

2. The LED of claim 1, wherein the second current path is coupled between
the substrate and the n-type semiconductor layer.

3. The LED of claim 2, wherein the second current path forms a non-ohmic
contact with the n-type semiconductor layer.

4. The LED of claim 2, wherein the second current path comprises an
electrically conductive material.

18. The LED of claim 15, wherein the bonding layer comprises at least one
of Ti/Au, Ti/Al, Ti/Pt/Au, Cr/Au, Cr/Al, Al, Au, Ni/Au, Ni/Al, or
Cr/Ni/Au.

19. The LED of claim 15, wherein the thickness of the bonding layer is in
a range from 0.5 to 10 μm.

20. The LED of claim 1, further comprising a p-electrode interposed
between the substrate and the LED stack, wherein the p-electrode
comprises an electrically conductive structure for guiding current
through the LED stack such that the light emitted directly underneath the
n-electrode is less than the light emitted from other areas of the LED
stack.

21. The LED of claim 20, wherein the current-guiding structure comprises
first and second contacts, the first contact having a higher electrical
resistance than the second contact.

22. A light-emitting diode (LED), comprising: an n-electrode; an LED
stack for emitting light disposed below the n-electrode, wherein the LED
stack comprises an n-type semiconductor layer coupled to the n-electrode
and a p-type semiconductor layer disposed below the n-type semiconductor
layer; and a p-electrode disposed below the p-type semiconductor layer,
wherein the p-electrode comprises an electrically conductive structure
for guiding current through the LED stack such that the light emitted
directly underneath the n-electrode is less than the light emitted from
other areas of the LED stack.

23. The LED of claim 22, wherein the current-guiding structure comprises
first and second contacts, the first contact having a higher electrical
resistance than the second contact.

24. The LED of claim 23, wherein the first contact comprises a barrier
metal layer.

25. The LED of claim 23, wherein the first contact comprises multiple
metal layers.

29. The LED of claim 23, wherein the second contact comprises an
omni-directional reflective (ODR) system.

30. The LED of claim 23, wherein an area of the second contact is larger
than an area of the first contact.

31. The LED of claim 23, wherein the first contact is disposed in a
voided space of the second contact.

32. The LED of claim 23, wherein the resistance of the first contact is
at least double the resistance of the second contact.

33. The LED of claim 22, further comprising a substrate disposed under
and coupled to the p-electrode.

34. The LED of claim 22, wherein the LED is a vertical light-emitting
diode (VLED).

35. A light-emitting diode (LED), comprising: a substrate; a p-electrode
disposed above the substrate; an LED stack for emitting light disposed
above the p-electrode, wherein the LED stack comprises: a p-type
semiconductor layer coupled to the p-electrode; and an n-type
semiconductor layer disposed above the p-type semiconductor layer,
wherein the LED stack provides a first current path for the LED; an
n-electrode disposed above the n-type semiconductor layer, wherein the
p-electrode comprises an electrically conductive structure for guiding
current through the LED stack such that the light emitted directly
underneath the n-electrode is less than the light emitted from other
areas of the LED stack; and a second current path for the LED different
from the first current path, wherein the second current path is coupled
between the substrate and the n-type semiconductor layer and wherein the
second current path comprises a protective device disposed adjacent the
n-type semiconductor layer.

36. The LED of claim 35, wherein the current-guiding structure comprises
first and second contacts, the first contact having a higher electrical
resistance than the second contact.

37. The LED of claim 35, wherein the protective device comprises at least
one of ZnO, ZnS, TiO2, NiO, SrTiO3, SiO2, Cr2O3,
or polymethyl-methylacrylate (PMMA).

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of co-pending U.S.
patent application Ser. No. 13/161,254, filed Jun. 15, 2011, which is a
continuation of U.S. patent application Ser. No. 12/823,866 filed Jun.
25, 2010, now issued as U.S. Pat. No. 8,003,994, which is a division of
U.S. patent application Ser. No. 12/136,547 filed Jun. 10, 2008, now
issued as U.S. Pat. No. 7,759,670, which claims the benefit of U.S.
Provisional Patent Application Ser. No. 60/943,533, all of which are
incorporated herein by reference in their entirety.

[0005] During the fabrication of light-emitting diodes (LEDs), an
epitaxial structure of an "LED stack" including layers of p-doped GaN and
n-doped GaN, for example, may be formed. FIG. 1 illustrates such an
example of a conventional LED device 102, having an n-doped layer 106 and
a p-doped layer 110 separated by a multi-quantum well (MQW) layer 108.
The device 102 is typically deposited on a carrier/growth-supporting
substrate (not shown) of suitable material, such as c-plane silicon
carbide (SiC) or c-plane sapphire, and bonded via a bonding layer 204 to
a thermally and electrically conductive substrate 101. A reflective layer
202 may enhance brightness. Voltage may be applied between the n-doped
layer 106 and p-doped layer 110 via an n-electrode 117 and the conductive
substrate 101, respectively.

[0006] In some cases, it may be desirable to control the amount of current
through the n-electrode 117 to the substrate 101, for example, to limit
power consumption and/or prevent damage to the device 102. Therefore, an
electrically insulative layer 206 may be formed below the p-doped layer
110, in the reflective layer 202, to increase contact resistance below
the n-electrode 117 and block current. The insulative layer 206 may be
similar to the current-blocking layer described in Photonics Spectra,
December 1991, pp. 64-66 by H. Kaplan. In U.S. Pat. No. 5,376,580,
entitled "Wafer Bonding of Light Emitting Diode Layers," Kish et al.
teach etching a patterned semiconductor wafer to form a depression and
bonding the wafer to a separate LED structure such that the depression
creates a cavity in the combined structure. When the combined structure
is forward biased by applying voltage, current will flow in the LED
structure, but no current will flow through the cavity or to the region
directly beneath the cavity since air is an electrical insulator. Thus,
the air cavity acts as another type of current-blocking structure.

[0007] Unfortunately, these approaches to current guiding have a number of
disadvantages. For example, the electrically insulative layer 206, the
air cavity, and other conventional current-blocking structures may limit
thermal conductivity, which may increase operating temperature and
compromise device reliability and/or lifetime.

[0008] Furthermore, a conventional LED device, such as the device 102 of
FIG. 1, may be susceptible to damage from electrostatic discharge (ESD)
and other high voltage transients. ESD spikes may occur, for example,
during handling of the device whether in fabrication of the LED device
itself, in shipping, or in placement on a printed circuit board (PCB) or
other suitable mounting surface for electrical connection. Overvoltage
transients may occur during electrical operation of the LED device. Such
high voltage transients may damage the semiconductor layers of the device
and may even lead to device failure, thereby decreasing the lifetime and
the reliability of LED devices.

[0009] Accordingly, what is needed is an improved technique for guiding
current through an LED device.

SUMMARY OF THE INVENTION

[0010] Embodiments of the present invention generally provide techniques
and devices for guiding current in semiconductor devices, such as
light-emitting diodes (LEDs).

[0011] One embodiment of the present invention provides an LED. The LED
generally includes a substrate; an LED stack for emitting light disposed
above the substrate, wherein the LED stack comprises a p-type
semiconductor layer and an n-type semiconductor layer; an n-electrode
disposed above the n-type semiconductor layer; and an electrically
conductive material coupled between the substrate and the n-type
semiconductor layer and forming a non-ohmic contact with the n-type
semiconductor layer.

[0012] Another embodiment of the present invention provides an LED. The
LED generally includes a substrate; an LED stack for emitting light
disposed above the substrate, wherein the LED stack comprises a p-type
semiconductor layer and an n-type semiconductor layer; an n-electrode
disposed above the n-type semiconductor layer; a protective device
disposed above the n-type semiconductor; and an electrically conductive
material coupled between the substrate and the protective device.

[0013] Yet another embodiment of the present invention provides an LED.
The LED generally includes a substrate; a p-electrode disposed above the
substrate and having first and second contacts, wherein the first contact
has a higher electrical resistance than the second contact; an LED stack
for emitting light disposed above the p-electrode, wherein the LED stack
comprises a p-type semiconductor layer coupled to the p-electrode and an
n-type semiconductor layer; and an n-electrode disposed above the n-type
semiconductor layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] So that the manner in which the above recited features of the
present invention can be understood in detail, a more particular
description of the invention, briefly summarized above, may be had by
reference to embodiments, some of which are illustrated in the appended
drawings. It is to be noted, however, that the appended drawings
illustrate only typical embodiments of this invention and are therefore
not to be considered limiting of its scope, for the invention may admit
to other equally effective embodiments.

[0016]FIG. 2 illustrates an example LED device with a current-guiding
structure in accordance with embodiments of the present invention.

[0017]FIG. 3 illustrates an equivalent circuit to the LED device of FIG.
2.

[0018]FIG. 4 illustrates an example LED device with a second current path
in accordance with embodiments of the present invention.

[0019]FIG. 5 illustrates an equivalent circuit to the LED device of FIG.
4.

[0020]FIG. 6 illustrates an example LED device with a current-guiding
structure and a second current path in accordance with embodiments of the
present invention.

[0021]FIG. 7 illustrates an equivalent circuit to the LED device of FIG.
6.

[0022]FIG. 8 illustrates an example LED device with a second current path
with a protective device in accordance with embodiments of the present
invention.

[0023]FIG. 9 illustrates an example LED device with a current-guiding
structure and a second current path with a protective device, in
accordance with embodiments of the present invention.

[0024]FIG. 10 illustrates an example LED device with a second current
path, in chip form, in accordance with embodiments of the present
invention.

[0025] FIG. 11 illustrates an example LED device with a second current
path, in package form, in accordance with embodiments of the present
invention.

[0026]FIG. 12 illustrates an example current vs. voltage (I-V) graph
comparing LED devices with and without a second current path.

[0027]FIG. 13 illustrates an example graph of electrostatic discharge
(ESD) level and corresponding survival rate of LED devices with and
without a second current path.

DETAILED DESCRIPTION

[0028] Embodiments of the present invention generally provide techniques
for controlling current flow through a semiconductor device, such as a
light-emitting diode (LED). The control may be via a current-guiding
structure, a second current path, or a combination thereof.

[0029] Hereinafter, relative terms such as "above," "below," "adjacent,"
"underneath," are for convenience of description and are typically not
intended to require a particular orientation.

An Exemplary Current-Guiding Structure

[0030]FIG. 2 illustrates an example light-emitting diode (LED) device
with a current-guiding structure in accordance with embodiments of the
present invention. The device may include a device structure known as an
LED stack comprising any suitable semiconductor material for emitting
light, such as AlInGaN. The LED stack may include a heterojunction
composed of a p-type semiconductor layer 110, an active layer 108 for
emitting light, and an n-type semiconductor layer 106. The LED stack may
have a top surface 119, which may be roughened as shown in FIG. 2. The
LED device may comprise an n-electrode 117 formed on the top surface 119,
the n-electrode 117 being electrically coupled to the n-type
semiconductor layer 106, and a p-electrode (a reflective layer 202 and a
barrier metal layer 208 may function as the p-electrode) on the p-type
semiconductor layer 110.

[0031] Disposed adjacent to the p-type layer 110 may be a reflective layer
202 interjected by a barrier metal layer 208 forming low contact
resistance areas 213 and a high contact resistance area 211,
respectively. For some embodiments, the volume of the low contact
resistance area 213 is larger than the high contact resistance area 211.
Electrically conductive, but having a higher electrical resistance than
the low contact resistance area 213, the high resistance contact area 211
may be formed utilizing a metallic material, as described below. The use
and careful manipulation of areas with different levels of contact
resistance may serve to direct the current to emit light from the active
layer in desired areas, such as light emission mainly from the active
layer in areas that are not disposed underneath the n-electrode 117 for
enhanced light emission.

[0032] In this manner, the LED device of FIG. 2 with the fully
electrically conductive current-guiding structure may have greater
thermal conductivity when compared to traditional LED devices with
conventional current-blocking or other current-guiding structures, such
as the LED device of FIG. 1 with an electrically insulative layer 206.
Therefore, the LED device of FIG. 2 and other embodiments of the present
disclosure with an electrically conductive current-guiding structure may
enjoy decreased operating temperature and increased device reliability
and/or lifetime when compared to such traditional LED devices.

[0033]FIG. 3 illustrates an equivalent circuit 300 for the LED device of
FIG. 2. As illustrated, the equivalent circuit 300 includes resistors
RL 302 and RH 304 in parallel that model the equivalent
resistances of the high and low resistance contact areas 211, 213 of FIG.
2. Although only one resistor is shown for the low contact resistance
area 213, this resistor RL 302 may represent the lumped equivalent
of one or more parallel low contact resistance areas, such as the two
areas 213 shown in FIG. 2. In a similar manner, the resistor RH 304
may represent the lumped equivalent of one or more parallel high contact
resistance areas 211. For some embodiments, the equivalent high contact
resistance may be at least two times the equivalent low contact
resistance. As illustrated, the parallel resistors, RL 302 and
RH 304, are in series with a diode 306 representing an ideal LED
with no series resistance.

[0034] One or more layers of a substrate 201 may be disposed adjacent to
the p-electrode (composed of the reflective layer 202 and the barrier
metal layer 208 in FIG. 2). The substrate 201 may be electrically
conductive or semi-conductive. For some embodiments, the substrate 201
may be thermally conductive. A conductive substrate may be a single layer
or multiple layers and may comprise, for example, metal or metal alloys,
such as Cu, Ni, Ag, Au, Al, Cu--Co, Ni--Co, Cu--W, Cu--Mo, Ge, Ni/Cu, or
Ni/Cu--Mo. Such a substrate may be deposited using any suitable thin film
deposition technique, such as electrochemical deposition (ECD),
electroless chemical deposition (Eless CD), chemical vapor deposition
(CVD), metal organic CVD (MOCVD), and physical vapor deposition (PVD).
For some embodiments, a seed metal layer may be deposited using
electroless chemical deposition, and then one or more additional metal
layers of the substrate 201 may be deposited above the seed metal layer
using electroplating. A semi-conductive substrate may comprise a single
layer or multiple layers and may be composed of silicon (Si) or silicon
carbide (SiC), for example. The thickness of the substrate 201 may range
from about 10 to 400 μm.

[0036] For some embodiments, the low contact resistance area(s) 213 may
comprise an omni-directional reflective (ODR) system. An ODR may comprise
a conductive transparent layer, composed of such materials as indium tin
oxide (ITO) or indium zinc oxide (IZO), and a reflective layer. The ODR
may be interjected by a current blocking structure or other suitable
structure in an effort to direct the current. An exemplary ODR system is
disclosed in commonly-owned U.S. patent application Ser. No. 11/682,780,
entitled "Vertical Light-Emitting Diode Structure with Omni-Directional
Reflector" and filed on Mar. 6, 2007, herein incorporated by reference in
its entirety.

[0037] The n-electrode 117 (also known as a contact pad or n-pad) may be a
single metal layer or multiple metal layers composed of any suitable
material for electrical conductivity, such as Cr/Au, Cr/Al, Cr/Al,
Cr/Pt/Au, Cr/Ni/Au, Cr/Al/Pt/Au, Cr/Al/Ni/Au, Ti/Al, Ti/Au, Ti/Al/Pt/Au,
Ti/Al/Ni/Au, Al, Al/Pt/Au, Al/Pt/Al, Al/Ni/Au, Al/Ni/Al, Al/W/Al,
Al/W/Au, Al/TaN/Al, Al/TaN/Au, Al/Mo/Au. The thickness of the n-electrode
117 may be about 0.1˜50 μm. The n-electrode 117 may be formed by
deposition, sputtering, evaporation, electroplating, electroless plating,
coating, and/or printing on the top surface 119 of the LED stack.

[0039] As illustrated in FIG. 2, protective layers 220 may be formed
adjacent to the lateral surfaces of the LED device. These protective
layers 220 may serve as passivation layers in an effort to protect the
LED device, and especially the heterojunction, from electrical and
chemical conditions in the environment.

[0040] The high and low contact resistance areas may be formed, for
example, by depositing one or more layers serving as the reflective layer
202 by any suitable process, such as electrochemical deposition or
electroless chemical deposition. Areas designated for high contact
resistance areas 211 may be removed in the reflective layer 202 by any
suitable process, such as wet etching or dry etching. Following removal
of the designated areas, a barrier metal layer 208 may be formed in the
voided spaces within the reflective layer 202. For some embodiments as
illustrated in FIG. 2, the barrier metal layer 208 composing the high
contact resistance area(s) 211 may fill in the voided spaces within the
reflective layer 202 and cover the reflective layer.

[0041] For some embodiments, the LED stack top surface 119 may be
patterned or roughened to increase light extraction when compared to LED
stacks with a smooth top surface. The top surface 119 may be patterned or
roughened using any suitable technique (e.g., wet or dry etching).

[0042] For some embodiments, the current-guiding structure described
herein may be combined with a second current path as illustrated in FIGS.
6 and 9. This second current path is described in greater detail below,
with reference to FIG. 4.

An Exemplary Second Current Path

[0043]FIG. 4 illustrates an example light-emitting diode (LED) device 400
with a second current path 402 in accordance with embodiments of the
present invention. As illustrated, the LED device 400 may include a
substrate 201, a p-electrode 207 disposed above the substrate 201, an LED
stack 104 disposed above the p-electrode 207, and an n-electrode 117
disposed above the LED stack 104. The substrate 201 may be thermally
conductive and electrically conductive or semi-conductive, as described
above. The LED stack 104 may include a heterojunction, which may comprise
a p-type semiconductor layer 110, an active layer 108 for emitting light,
and an n-type semiconductor layer 106. A second electrically conductive
material 411 may be coupled to the substrate 201 and to the n-type
semiconductor layer 106 to form a non-ohmic contact 412 with n-type
semiconductor layer 106 in an effort to provide a second current path 402
between the substrate 201 and the n-semiconductor layer 106. The second
conductive material 411 may be formed via any suitable process, such as
e-beam deposition, sputtering, and/or printing.

[0044] As illustrated, an electrically insulative layer 404 may separate
the second conductive material 411 and at least a portion of the LED
stack 104. The insulative layer 404 may comprise any suitable
electrically insulative material, such as SiO2, Si3N4,
TiO2, Al2O3, HfO2, Ta2O5, spin-on glass
(SOG), MgO, polymer, polyimide, photoresistance, parylene, SU-8, and
thermoplastic. For some embodiments, the protective layers 220 may serve
as the insulative layer 404.

[0045] As described above, the substrate 201 may be a single layer or
multiple layers comprising metal or metal alloys, such as Cu, Ni, Ag, Au,
Al, Cu--Co, Ni--Co, Cu--W, Cu--Mo, Ge, Ni/Cu and Ni/Cu--Mo. The thickness
of the substrate 201 may be about 10 to 400 μm.

[0046]FIG. 5 illustrates an equivalent circuit 500 for the LED device of
FIG. 4. As illustrated, the equivalent circuit 500 includes two parallel
current paths. The first current path includes an equivalent resistor
RL 502 in series with an ideal LED 504 forming a forward current
path from the substrate 201 to the n-electrode 117. The second current
path 402 is represented by a bidirectional transient voltage suppression
(TVS) diode 506. The TVS diode 506 may operate similar to two opposing
zener diodes connected in series and may serve to protect the resistor
502 and ideal LED 504 from high voltage transients. The TVS diode 506 can
respond to over-voltages faster than other common over-voltage protection
components (e.g., varistors or gas discharge tubes) making the TVS diode
506 useful for protection against very fast and often damaging voltage
transients, such as electrostatic discharge (ESD). The second conductive
material 411 of FIG. 4 may form the TVS diode 506 of FIG. 5. The second
conductive material 411 may shunt excess current in either direction when
the induced voltage exceeds the zener breakdown potential.

[0047]FIG. 6 illustrates another example LED device with a second current
path 402 in accordance with embodiments of the present invention. As
illustrated, an LED device with a second current path 402 may also
include the current-guiding structure resulting from separate high and
low resistance contact areas 211, 213. For example, these different
contact areas may be formed by the interjection of the reflective layer
202 by the barrier metal layer 208, as described above with respect to
FIG. 2.

[0048]FIG. 7 illustrates an equivalent circuit 700 to the LED device of
FIG. 6. As illustrated, the single equivalent resistance RL 502 of
FIG. 5 is replaced by the parallel combination of RH 304 and RL
302 to represent adjacent high and low contact resistance areas 211, 213
of FIG. 6. The remainder of the circuit 700 is equivalent to the circuit
500 of FIG. 5. In other words, the LED device of FIG. 6 may have the
advantages of current guiding and transient suppression.

[0049]FIG. 8 illustrates another example LED device with a second current
path 402 in accordance with embodiments of the present invention. In this
embodiment, a protective device 810 is formed in the second current path
402. As illustrated, the protective device 810 may be formed on the
n-type semiconductor layer 106 and may serve to increase the level of
transient voltage protection or the current capability, thereby
increasing the reliability and/or lifetime of the LED device. The
protective device 810 may comprise any suitable material, such as ZnO,
ZnS, TiO2, NiO, SrTiO3, SiO2, Cr2O3, and
polymethyl-methylacrylate (PMMA). The thickness of the protective device
810 may range from about 1 nm to 10 μm.

[0050] As illustrated in FIG. 9, an LED device with a second current path
402 and a protective device 810, as shown in FIG. 8, may also include a
current-guiding structure resulting from separate high and low resistance
contact areas 211, 213. For example, these different contact areas may be
formed by the interjection of the reflective layer 202 by the barrier
metal layer 208, as described above with respect to FIG. 2.

[0051]FIG. 10 illustrates an example LED device with a second current
path, in chip form, in accordance with embodiments of the present
invention. As illustrated, a bonding metal layer 1002 may be deposited
for electrical connection above the protective device 810 in the second
current path. The bonding layer 1002 may comprise any suitable material
for electrical connection, such as Al, Au, Ti/Au, Ti/Al, Ti/Pt/Au, Cr/Au,
Cr/Al, Ni/Au, Ni/Al, or Cr/Ni/Au. The thickness of the bonding layer 1002
may range from 0.5 to 10 μm. For some embodiments, the n-electrode 117
may be extended to allow bonding to a package, as described below with
respect to FIG. 11.

[0052] FIG. 11 illustrates the LED device of FIG. 10 in package form, in
accordance with embodiments of the present invention. As illustrated, the
substrate 201 may be bonded to a common package anode lead 1102. The
bonding layer 1002 may be coupled to the anode lead 1102 via a bonding
wire 1104 attached to the bonding metal 1002, thereby forming the second
current path. The n-electrode 117 may be coupled to a cathode package
lead 1106 via another bonding wire 1108.

[0053]FIG. 12 is a graph 1200 plotting example I-V curves 1204, 1202 of
an LED device with and without a second current path, respectively. As
illustrated by the I-V curve 1204, the second current path may allow an
LED device to withstand higher voltage without an excessive amount of
current, which may prevent damage and/or prolong device life.

[0054]FIG. 13 illustrates an example graph 1300 of ESD voltage and
corresponding survival rate of LED devices with and without a second
current path. In the illustrated scenario, LED devices 1304, 1306, 1308,
1310, 1312 without a second current path pass the test with various
survival rates at various ESD voltages. In contrast, LED devices 1302
with the second current path pass at a rate at or near 100%, even at
higher ESD voltage levels greater than 2000 V.

[0055] While the current-guiding structures described herein have
advantages that apply to vertical light-emitting device (VLED) devices,
those skilled in the art will recognize that such advantages generally
apply to most semiconductor devices. Therefore, the structures described
herein may be used to advantage to form low resistance contacts and/or
transient suppressors for any type of semiconductor device having a p-n
junction.

[0056] While the foregoing is directed to embodiments of the present
invention, other and further embodiments of the invention may be devised
without departing from the basic scope thereof, and the scope thereof is
determined by the claims that follow.